The present invention relates to multiplexing a plurality of analog signals onto a single output channel and particularly relates to a multiplexer useful in generating a serial video signal from a plurality of parallel bi-directional scanning thermal imaging sensors without the requirement of using a vidicon or other electro-optical transducers.
Thermal imaging systems are experiencing wider application and development for many reasons including the ability of such systems to form images without the aid of visible light. In one particular example of a thermal imaging system, a picture is constructed by causing a column of IR detectors to scan across an observation scene. In many applications, this scanning is accomplished utilizing a mirror which first scans in one direction and then scans back in the return direction in alternate interleaved scanning positions to form one entire picture for each such bi-directional scan. The picture must then be transformed from the IR frequency range to the visible spectrum so that it may be observed. This has been accomplished in prior systems by utilizing the signals from the IR detectors to activate a column of light emitting diodes which are arranged to transmit the generated light to the back face mirror which is also a reflective surface. The reflective back surface of the mirror is then observed by the system operator. However, substantial difficulties arise in such systems when more than one observer must observe the scene. In such systems, complex optical devices including mirrors, prisms, etc. which involve complex alignment problems have been utilized. In addition, vidicons or other electro-optical transducers have been required which have further increased the complexity of such systems.
Although these electro-mechanical problems and complexities exist, an all-electronic display system which utilizes CRTs or other electronic display medium has also raised problems. One difficulty, particularly apparent in bi-directional scanning systems, is that in order to convert the plurality of video signals appearing on the parallel channels from the IR detectors, it is necessary to convert these parallel signals into a single, serial video signal which is required in order to incorporate a CRT display. Conventional parallel-to-serial multiplexers are not acceptable particularly when the system utilizes bi-directional scanning in an interlaced line scanning and display system, because the scanning is continuous and all of the video input channels cannot be outputted simultaneously and still be serial in nature. Consequently, each of the video samples in the conventional arrangement is outputted serially on the single output line at a slightly different time and thus represents a slightly different look angle for the detector column. The resultant picture thus appears slightly skewed. Such skewing becomes a particular problem in bi-directional scanning because the slant occurs in one direction for one direction of scanning and in the opposite direction for the opposite direction of scanning resulting in substantial distortion in the resultant CRT picture.
The present invention alleviates these problems by providing a novel multiplexer which has particular use in bi-directional scanning thermal imaging systems.